Ferritic,martensitic and ferriteaustenitic chromium steels with reduced tendency to 475 c.-embrittlement



STEELS WITH REDUCED TENDE Filed May 4, 1967 R G. LAGNEBORG 3,499,802OMIUM ITTLEMENT 3 Sheets-Sheet 1 FERRITE-AUSTENITIC can NCY TO 475C.-EMBR I I I h l I v G.) g 57; cv l Go I ,29 I l I U) .E I I a:

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FERRITIC, MARTENSITIC AND FERRITE-AUSTENITIG CHROMIUM -EMBRITTLEMENTlSgEEELS WITH REDUCED TENDENCY '10 475C 3 Sheets-Sheet. 2

Filed May 4,

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March 10, 1970 R. G. LAGNEBORG 3,499,802

FERRITIC, MARTENSITIC AND FERRITE-AUSTENITIC CHROMIUM STEELS WITHREDUCED TENDENCY T0 475C.-EMBRITTLEMENT Filed May 4, 1967 3 Sheets-Sheet3 Fig.3

United States Patent M US. Cl. 14837 3 Claims ABSTRACT OF THE DISCLOSURE475 embrittlement in ferritic chromium steels containing upwardly from13% chromium is prevented by including in the steel composition anoptimumrather smalladdition of aluminum or cobalt or both.

Ferritic chromium steels with chromium contents exceeding 13% show atendency to so-called 475 C.- embrittlement. This embrittlement appearswhen these steels are being held at, or are slowly cooling down through,the temperature range of 400-550 C. This type of embrittlementwhich isclearly distinguished from the embrittlement caused by precipitation ofsigma phase at higher temperatures up to 700 C.has been described e.g.in the following literature references:

1) Identification of the Precipitate Accompanying 885 F. Embrittlementin Chromium Steels, R. M. Fisher, E. I. Dulis, K. G. Carroll, J. Metals197 (1953), p. 690.

(2) Further Studies of the Iron-Chromium System, R. 0. Williams, TransAIME (1958) 497, vol. 212.

(3) Ueber das Verhalten von Stahlen mit 11 bis 20% Cr nach Auslagerungim Bereich der 475-Versprodung, E. Baerlecken, H. Fabritius, Stahl u.Eisen 78 (1958), p. 1389.

(4) Effect of 500 C. Ageing on the Deformation Behavior of anIron-Chromium Steel Alloy, M. I. Marcinkowski, R. M. Fisher, A. Szirmae,Trans AIME 230 (1964), p. 676.

(5) Influence of Alloying Elements of the Impact Transition Behaviour of12% Chromium Steels Aged at 900 F., E. J. Whittenberger, E. R. Rosenow,Trans ASM 48 (1956) p. 391.

(6) Metallography of an Iron-21% Chromium Alloy Subjected to 475 C.Embrittlement, M. J. Blackburn, J. Nutting, 1. Iron Steel Inst, 202(1964), p. 610.

In these literature references some different theories are discussedregarding the cause of the phenomena which are usually designated 475 C.embrittlement. However, it cannot yet be considered that the cause ofthese phenomena has been made clear, nor is it considered possible tocompensate foror to prevent-those phenomena by adding alloyingcomponents or in any other way. However, some of the investigationswhich are described or related in the above literature referencesindicate that the 475 C. embrittlement is connected with a precipitationof a ferrite, rich in chromium-probably, with about 80% chromiumin amiscibility gap in the system of ironchromium. The same phenomena ofembrittlement appear in the strongly ferritic chromium steels; also, inchromium steels containing ferrite, e.g. ferrite-austenitic steels andsuch steels which contain a phase similar to ferrite, e.g. martensite ortempered martensite.

According to the present invention it has now appeared that thedeterioration in properties, which follows a socalled 475 C.embrittlement of ferritic chromium steels 3,499,802 Patented Mar. 10,1970 containing more than 13% and up to 18% chromium, preferably15-17.5% chromium and steel of the corresponding composition containinga ferritic phase or a phase similar to ferrite, e.g. martensitic steelsand ferriteaustenitic steels can be compensated for or preventedeffectively by alloying the steel with aluminum and/or cobalt in anamount of at least 0.5% by wt., preferably at least 1% and especially atleast 1.5% and at most 4.0%, preferably 3% and especially at most 2 to2.5 wt. percent. The suitable upper limit of the proportion of aluminumand/or cobalt can be varied slightly depending on the remaining alloyingconstituents in the steel, and should in the cases of some martensiticsteels be limited with regard to the capability as austenite formers.

The addition of aluminum and/or cobalt to ferritic chromium steels tocompensate for or to prevent the deterioration of the properties of thesteel which is connected with 475 C. embrittlement can be generallyapplied to steels which besides iron and chromium also contain otheralloying components intentionally added or occurring as impurities, inso far as they do not abolish the advantageous effect of the addition ofaluminum and/ or cobalt. As examples of such components, the followingcan be mentioned.

Carbon usually is a part of ferritic chromium steels to an amount of atleast 0.0l -0.02% and in martensitic steels at least 0.15% by weight.The maximum carbon content varies with the chromium content and as forausteniteferritic steels it can be about 0.1%; as for ferritic steels,high in chromium, 0.20-0.25%, preferably 0.15%, and as for martensiticsteels 0.5%, preferably 0.4% by wt., the upper limits usually beingfixed by the risk of austenite formation at annealing temperature.

Silicon usually does not substantially impart brittleness, but it mayinfluence the position of the phase boundary or the miscibility gap. Themaximum silicon content can usually be chosen 0.6% and preferably 0.3%,the minimum content usually being 0.05%.

With regard to the risk of austenite formation the manganese contentshould be restricted to a maximum of 1.5%, preferably maximum 1% by wt.The content of nitrogen generally is at most 0.05%, but as for certainnitrogen-alloyed steels the content can amount to 0.20%. Averagenitrogen content lies between 0.005 and 0.03% by weight.

Molybdenum improves the strength at elevated temperatures. The additionof molybdenum should be restricted to 3-4% as a maximum. As for ferriticsteels and as for martensitic steels to a maximum 1.5% and minimum 0.5respectively, 0.25% for ferritic respectively, martensitic steels.

Nickel may be included in varying amounts depending on the type of thesteel. The nickel content in ferritic steels is usually defined tomaximum 2.5%, preferably 1%, sometimes to 3.5%, the corresponding amountfor martensitic and austenite-ferritic steels being maximum 2.5- 1%.However, nickel could also be eliminated from the composition.

Titanium, niobium and/or tantalum may be used in order to bind carbonand nitrogen as carbides and nitrides, preferably, up to a maximumcontent of 1% Ti and 1% Nb and/or tantalum.

Vanadium can be added to regulate the grain size preferably up to amaximum content of 0.5%. Tungsten can be added up to 1%.

According to the present invention an addition of aluminum and/or cobaltto steels inclined to 475 C. embrittlement compensates or prevents thedeterioration of the properties of the steel which follows thisphenomenon. Among the most important changes occasioned by 475' C.embrittlement should be mentioned an increased hardness,

some raising of the 0.2-limit, lowering the true tensile strength andfatigue strength, and especially decreasing contraction at fractureobtained in tensile test and a tendency to brittle fracture withoutnecking. From the point of view of this application, this embrittlementis usually the most important change following the phenomena called 475C. embrittlement and which appear in a temperature range of 400500 andespecially 440-500 C., this range to a certain extent being dependent onthe composition.

The contraction of steels inclined to 475 C. embrittlement usually isless than 15%. Steel with an addition of aluminum and/r cobalt accordingto the present invention usually shows a rather small decrease incontraction as measured on a static tensile test specimen and comparedto non-aged condition, i.e. material which is not heattreated at 475 C.

In the following the invention will be described in more detail withembodiments referring to the appended drawing, in which FIGS. 1 and 2are graphic charts illustrating the properties of the alloys of thepresent invention; and

Cir

alloy A. However, at times exceeding 200 hours, the yield strength isgreater for the alloy B and after tempering for 1000 hours 0 is about 20kg./mm. greater for B than for A. This shows that the 475 C.embrittlement cannot alone stand for the increase in yield strength.

Finally, in FIG. 3 the appearances of the breaks of the static tensiletest specimens are shown, which specimens were used in the experimentsaccording to the embodiment. The static tensile test specimens a-f wereproduced on alloy B with an addition of aluminum to preventembrittlement according to the invention, while the test specimens g-mwere produced on alloy B, which consisted of the same material butwithout this aluminum addition. The ageing times in hours at 475 C. wereas follows: a, g=0, b, 11:10, 0, i=50, d, k=200, e, l=500, f, m=1000.

It is apparent from the figures that the test specimens of steel withaddition of aluminum according to the invention showed a ductile breakwith a considerable contraction even at the longest ageing time.

As additional examples of the composition of similar steel alloys whichare resistant to 475 C. embrittlement, the following can be mentioned:

0 Si Mn P max. S max. Cr A1 C0 N max Fe 0.05 0.3 0.3 0, 03 0.03 17.0 2.5 0.03 Bal. 0.05 0.3 0.3 0.03 0.03 15. 0 2.0 a 0.03 Bal. 0. 05 0.3 0.30. 03 0. 03 17.0 a. 2.0 0. 03 Bal.

FIG. 3 is a photograph of specimens after testing.

Alloys with the following compositions were aged for FIGS. 1, 2 and 3 ofWhlCh the results are related from 2000 hours and strll had the desiredproperties:

C Si Mn P max. S max. Cr Al N max. Fe

0. 036 o. 3 0. 3 0. 03 0. 03 16. o 2. 2 0. 055 Bal. 0. 033 0. 3 0. 3 0.03 0. 03 16. 2 1. 0 0. 046 Bal' 0. 01s 0. 3 0. 3 u. 03 0. 03 14. 9 2v 90. 025 B211. 0. 02s 0. 3 0.3 0. 03 0. 03 15. 0 1. 1 0. 025 Bal.

experiments with two steel alloys A and B, both comprise 17.4% chromiumand the latter also 1.9% aluminum. Both the alloys were produced in aninduction furnace with an atmosphere of argon gas. The startingmaterials were two high-vacuum melted materials, to wit, pure iron andan iron-chromium alloy comprising 30% chromium. The chemical compositionin percentage by weight of the steel alloys A and B is shown in Table I.

Regarding these four compositions, I have heat-treated the specimens at1050 C. to achieve a grain size of 150p. which is most risky at 475 C.embrittlement. The contraction then was 4050%. The specimen (1)(C=0.036)=40% contraction (2) (C=0.033)=47% contraction (3)(C=0.018)=47% contraction (4) (C=0.025)=% contraction TABLE I Aldissolved in C M11 P S Or the steel N Fe A O. 007 0. 04 O. 003 0. 01717. 1 0. 01 Bal. B 0. 006 0. 04 0. 009 0. 019 17. 1 1. 88 0. 03 B211.

Besides the contents stated, the alloys A and B contained only iron savefor insignificant amounts of impurities.

The alloys were hot-worked to 15 mm. 5 and then coldworked to 11 mm. Thegrain size of the materials was 70a. Static tensile test specimens wereproduced and aged at 475 C. for periods of 10, 50, 200, 500 and 1000hours. At the following tension test c true tensile strength, andcontraction at fracture were determined.

The results for the alloys A and B are presented in FIGS. 1 and 2 ofwhich the first shows contraction at fracture 4 in percent after ageingat 475 C., and the latter shows true tensile strength and a also afterageing at 475 C. As for the alloy A the contraction at fracture israpidly decreased, and already after an ageing period of about 50 hoursthe fracture changes from ductile fracture to brittle fracture withoutnecking. The contraction decreases insignificantly for alloy B duringthe first 200 hours and then it seems to assume a constant value ofabout 65%. FIG. 2 shows that the true tensile strength for the alloy Adecreases rapidly with the tempering time which reflects the increasingtendency to brittle fracturewhile the alloy B does not show such adecrease. With short tempering times 0 is somewhat greater for the Asexamples of conventional steel alloys the resistance to 475 C.embrittlement of whichaccording to the present invention-can beincreased by addition of aluminum and/or cobalt there can be mentionedsteels with compositions corresponding to the standard specifications:A181 430, 434, 446, 442, 420, 431, 440A, 440B.

Examples of improved steels of the ferrite-austenitic type are thefollowing:

C=max 0.15% (0.2%) Cr=18 a 2026% Si=01% Al and/ or Co =0.5-4%

l3-18% and preferably 15-17.5% chromium are objects meant to be exposedto the temperatures which cause embrittlement in a long time, as morethan, 100 hours and particularly more than 1000 hours, especially to400-550 C. and in particular 440-500 C. When used, the objects are alsoexposed to mechanical strain, eventually combined With oxidizingconditions.

The present invention is particularly applicable for objects of which itis demanded--when used-that the material has no tendency to brittlefracture.

Examples of applications are for purposes where there is demanded acontraction at fracture of at least 20% and preferablyv at least 30%based on experience. Among such purposes of applications are parts ofgas or steam turbines of different kinds; jet engines; compressors; andsimilar (especially movable or rotating) parts of them, i.e. buckets andthe like, especially if they are exposed during operation to acalculated tensile stress of at least 5 kg./mm.

Other examples are reaction vessels, conduits and similar articles whichare used in chemical processes or the like and are exposed, or run therisk of being exposed, to temperatures within the range given above,especially if the material at the same time is exposed to tensilestresses of at least 5 kg./mm. i.e. tangential stresses in the tubeorcontainer walls, caused by an inner gauge pressure.

Further objects are exhaust pipes of stationary and mobileestablishments; heat exchangers; and similar articles, especially ifthey are exposed to stable or varying tensile stresses. These stressesusually are risky if they normally amount to at least 5 kg./mm. causedi.e. by an inner gauge pressure, by vibrations, etc.

According to the present invention, the steels to which aluminum and/orcobalt is added are usually suited for purposes of application as thosementioned above, of which a contraction at fracture (by tensile testing)of at least 20% and preferably at least 30% is demanded; also, when thesteels are exposed to a temperature of 425- 550" C. during at least 200hours and preferably at least 1000 hours. (Method for testing: Bytensile testing of ferritic 17% Cr steel with a ferrite grain size 70;;aged at 475 C. during at least 200 hours and preferably at least 1000hours a contraction of at least 30% must be measured out).

I claim:

1. Article of ferritic, ferrite-austenitic or martensitic chromium steelhavinga structural condition produced by ageing at temperatures between425 and 550 C. during more than 1000 hours and thereby shows acontraction of at least 30% at fracture as measured on a static tensiletest specimen thereof and comprising, in percentages by weight, thefollowing:

the total content of titanium, niobium, tantalum and vanadium being atmost 2%, balance Fe and normal impurities.

2. Article according to claim 1, containing 14% of at least one memberof the group consisting of Al and Co.

3. Article according to claim 2, containing 1.5-3.0% of said member.

References Cited UNITEDjftSTATES PATENTS 1,745,360 2/1930 Antoinette.

2,590,835 4/1952 lKirkby.

2,820,708 1/1958 Waxweiler -128 x 3,068,095 12/1962 i Anthony.

3,108,870 10/1963 Brady ...1 75 124 3,152,934 10/1964 Lula.

3,250,612 5/1966 Roy.

HYLAND BIZOR, Primary Examiner US. Cl. X.R. 75-126, 128

